The present invention relates to a control device of a hydrogen engine in which a hydrogen fuel and at least one of fossil fuels such as gasoline, gas oil, natural gas, etc are used, and more specifically a technical filed of a control device of a hydrogen engine which is equipped with a NOx trap catalyst disposed in an exhaust passage.
Conventionally, the engine for a vehicle is equipped with a three-way catalyst which purifies CO (carbon monoxide), HC (hydrocarbon), NOx (nitrogen oxide) and so on in an exhaust gas. However, since a lean-burn type of engine has been recently developed to improve the fuel economy, the above-described three-way catalyst whose purifying window is somewhat limited to a small range around a stoichiometric air-fuel ratio could not purify NOx properly in the exhaust gas including excessive oxygen for such a lean-burn engine.
Herein, the NOx tarp catalyst is known as a catalyst disposed in the exhaust passage which functions in such a manner that it traps (absorbs) NOx in the exhaust gas thereat when the air-fuel-ratio state of the exhaust gas is lean (in an oxygen excessive state), whereas it releases (discharges) the trapped NOx therefrom when the air-fuel-ratio sate of the exhaust gas is rich (in an oxygen deficient state). By using this NOx trap catalyst and by properly controlling the air-fuel ratio of the exhaust gas, NOx is trapped (absorbed) at the NOx trap temporarily in the lean state and then the trapped NOx is released (discharged) in the rich state for reacting on CO and HC which exist excessively in the exhaust gas, so that NOx can be properly restrained from being emitted outside. Namely, the NOx emission can be properly reduced for the lean bun engine.
Japanese Patent Laid-Open Publication No. 10-311235 discloses this kind of exhaust purification device of an engine. This exhaust purification device includes the NOx trap catalyst in the exhaust passage, and when the amount of tapped NOx exceeds a specified value, the air-fuel ratio is made rich and the trapped NOx is released thereby. Herein, this rich control is made in such a manner that the air-fuel ratio is made rich intensively at the beginning of NOx release and then the rich degree is gradually decreased. According to the above-described rich control, the NOx release and NOx purification with CO and HC can be properly achieved.
Meanwhile, a hydrogen engine using a hydrogen fuel instead of the above-described fossil fuels or a hybrid type of hydrogen engine using both the hydrogen fuel and the fossil fuel have been recently proposed for an environmental protection. Japanese Patent Laid-Open Publication No. 7-97906 discloses an exemplified engine of this hydrogen engine. This engine includes a hydrogen fuel injector for supplying the hydrogen fuel into the combustion chamber as well as a gasoline fuel injector for supplying the gasoline fuel, in which the fuel injectors are selectable in using based on the engine operating condition.
Herrin, the above-described hydrogen engine has a problem that there may occur an inappropriate burning, for example, when the rich air-fuel-ratio control is executed to release the trapped NOx from the NOx catalyst during the lean burn engine operation with the hydrogen fuel. Namely, since the hydrogen fuel has a higher combustible reaction speed than the fossil fuel, such a rich combustion of hydrogen may cause the inappropriate burning, resulting in improper vibrations or noises, like an engine knocking, occurring.
The present invention has been devised in view of the above-described problem, and an object of the present invention is to provide a control device of a hydrogen engine in which any improper vibrations or noises can be restrained from occurring when the rich air-fuel-ratio control is executed to release the trapped NOx from the NOx trap catalyst.